In the conventional receiver used in the spread spectrum communication system of the direct sequence type, power of a base band signal is detected, and an intermediate frequency (IF, hereinafter) signal is amplified by an automatic gain control (AGC, hereinafter) amplifier so that power of the base band signal is kept to be constant. Accordingly, the IF signal is excessively amplified in case the signal to noise (S/N, hereinafter) ratio of the desired wave is high, and electric power consumed in the AGC circuit becomes high.
Especially, when the spread spectrum communication system is applied to the cellular telephone, since the AGC amplifier is always operative even when the receiver is in the standby state, it is very important to reduce electric power consumed in the receiving circuit from a view point of extension of a lifetime of batteries of the cellular telephone. Accordingly, an uncomplicated method for reducing electric power consumed in the receiver has been strongly desired.
Moreover, in the conventional receiver, since power of the base band signal is controlled depending an power of all the waves including the desired wave received by an antenna in an assigned frequency band, power of the symbol rate signal of the desired wave which is outputted from the despreader is not necessarily kept to be constant. Accordingly, in case that the signal to noise (S/N, hereinafter) ratio of the desired wave is low, explaining concretely in case that the other transmitting wave or an interference wave exists in the same frequency band as that of the desired wave, or power of the desired wave received by the antenna is lower that of thermal noise power, there arises an apprehension that the desired wave cannot be amplified satisfactorily, or the sensibility of the receipt of message and the equality of the signal transmission will be deteriorated.